I needed an amplifier for EME use and was searching for TH327 or YL1050 at a decent cost
without any luck. I had a couple of GS23Bís and decided to try them on 1296MHz trying to
reach 1KW output. The 4CX1600U data sheet from Svetlana specifies the tube as good for
1300MHz with reduced ratings.

Click for Zoom

The amplifier described here is a good alternative to YL1050 and has very similar performance.
I strongly recommend modifying the tube for water-cooling. This will reduce or eliminate
thermal drift which is a common problem in amplifiers at this frequency. This amplifier
operates with grounded screen grid so the cathode is -500V in relation to ground. This means
that you must use a floating plate power supply. G3SEKís tetrode board can be configured for
this type of operation. Please read his web page at: http://www.ifwtech.co.uk/g3sek.

NOTE: Increasing the screen voltage to get higher plate current, thereby lowering
plate load impedance, results in a better match of the tube to the cavity in this design.
Raising the screen voltage to 525VDC with tube #1 installed in the PA, holding plate voltage
at value shown, power output increased to 1350W @ 39% efficiency.

Differences Amongst Tubes

The difference in gain and efficiency between tubes #1 and #2 existed before I converted #1
for water-cooling. Water-cooling did eliminate the thermal drift for tube #1 so I would
recommend it to anyone pushing the limit.

Thermal drift is measured as reduced output by internal changes in the tube causing de-tuning
of the cavity over 1 minute key-down.

Tube #2 reached max screen dissipation at 850W output so that is where I stopped.

Tube #3 did not perform as well as tube #1. The following was observed:

The difference between tubes is to be expected in 1296MHz PAs since even a very small
mechanical change will de-tune a cavity at this frequency. Anyone using GI7B, GS9B,
2C39, or 7289 triodes, all rated for 3000 MHz, experience the same problem. YL1050 and
TH327 PAs also exhibit the same variations at 23cm and it is considered normal. Everyone
should be aware of this fact before they get started. In fact, after testing over 20 tubes,
It appears that only one of somewhere from 5 to 7 are good for 23cm, so testing GS-23b
tubes for 23cm suitability is a necessity. Even so, those tubes which do not work at
1.3GHz (see discussion below) should operate at 432MHz, no problem!

Why Tubes Vary in 23cm Performance

The problem is that radiated heat from the plate plus the dissipated power in the screen
will mechanically change the screen, resulting in a change in plate/screen capacitance.
The ONLY way to minimize this problem is to have GOOD (preferably water) cooling to eliminate
the heating from the plate to the screen. It all has to do with the mechanical design and
assembly of the tube. If the grid cage and the screen cage windows are perfectly lined up the
resulting screen current will be very low and stability improved. A tube that shows
high grid and screen current doesn't have the grid & screen cage windows perfectly lined
up and some accelerated electrons hit the screen instead of passing through the window.

Many tubes have the cathode in the bottom and then a flat grid above it and then a flat
screen above the grid and so on. The GS23B has all the elements inside the tube in a coaxial
configuration. The cathode on GS23B is a cylinder in the middle and then the grid is another
cylinder outside the cathode and so on. This makes for a very compact tube with large element
surfaces and low inductance. This is a very common design in large tetrodes.

A tube that responds well to drive and has normal grid and screen current but low efficiency
probably has a small variation in the distance between the elements around the circle. Any
variation in this distance will accelerate the electrons in a non-symmetrical fashion around
the circle and they will arrive out of phase. There is nothing we can do about that from
outside the tube. This behavior is frequency dependent since the phase error gets smaller at
lower frequency. This is why a tube like this would work well at 70cm but not at 23cm.

Testing Conditions

Overall, maximum plate dissipation is a function of cooling, so it should not be a problem to
exceed it as long as the temperature is kept low.

None of the above critical parameters were exceded. These parameters are based on
continuous carrier so it might be possible to get even more output in intermittent ham
use, but I am not going to test it.

I have stress-tested the amp with tube #1 at 1200W output and everything looks good. I
increased the drive-power to 100W and the plate current went from 1020mA to 1120mA. The
efficiency increased. I got 200W more output for 300W more plate input. This is to be
expected since I am getting further away from the idle current of 200mA. I set my CW-keyer
to 20 seconds of code and 60 seconds pause with 100W drive. I operated the amp for more than
96 hours like this with no problems occurring. I am exceeding the screen dissipation by 10%
under these conditions, but I expect that the ratings on this tube are very conservative.

PA Construction - Materials

The PA shown here and used for testing was manufactured using brass and bronze. It did not
perform as expected until it was silver-plated. I strongly recommend, therefore, avoiding
these materials to make parts for this amplifier unless you are going to silver plate it.
To check further, I manufactured two more copies of this PA, one of aluminum and another of
unplated brass and bronze. The output from both of these amps as initially constructed was
300W lower output than the prototype at the same plate input power. After silver-plating the
brass-bronze amp, power increased to be identical to the prototype output. Clearly, a PA made
using brass-bronze requires silver plating for optimum performance. Bronze bushing material
was chosen for the anode cavity and grid sleeve because it has dimensions which simplify
fabrication of these parts. It appears that only if made of copper would this PA not require
silver plating. Aluminum should be avoided!